Solid electrolytic capacitor and method for producing the same

ABSTRACT

To provide a solid electrolytic capacitor where solid electrolyte is formed at the cut end part and at the masking part to a larger thickness than in other parts, where the adhesion of the solid electrolyte formed on the valve acting metal oxide film is improved, and whereby the capacitor is highly stabilized in various basic properties such as capacitance, dielectric loss (tan δ), leakage current and short circuit defective ratio and also in the reflow soldering heat resistance and moisture resistance load characteristics.  
     These solid electrolytic capacitor can be obtained by forming electrically conducting polymer on a dielectric film by specifying time for dipping of the surface of a valve actiong metal porous body with a solution containing a monomer and with a solution containing an oxidizing agent, time for vaporization of the solvent of the solution containing a monomer, and polymerization conditions after dipping of the solution containing an oxidizing agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on the provisions of 35 U.S.C. Article111(a) with claiming the benefit of filing dates of U.S. provisionalapplications Serial Nos. 60/216,518 filed on Jul. 6, 2000 and 60/257,303filed on Dec. 26, 2000 under the provisions of 35 U.S.C. 111(b),pursuant to 35 U.S.C. Article 119(e) (1).

TECHNICAL FIELD

[0002] The present invention relates to a solid electrolytic capacitorcomprising a porous valve-acting metal substrate having formed thereonan electrically conducting polymer as a solid electrolyte layer and alsorelates to a method for producing the solid electrolytic capacitor.

[0003] More specifically, it relates to a high-performance solidelectrolytic capacitor obtained by forming the solid electrolyte layersuch that the thickness at the cut surface of the substrate and at themasking boundary part is larger than the thickness in other parts, andalso relates to a method for producing the solid electrolytic capacitor,especially using a monomer solution and an oxidizing agent solution.

BACKGROUND ART

[0004] In the production of a solid electrolytic capacitor, as shown inFIG. 1, an oxide dielectric film layer 2 is generally formed on an anodesubstrate 1 comprising a metal foil subjected to an etching treatment tohave a large specific surface area, a solid semiconductor layer(hereinafter referred to as a “solid electrolyte”) 4 is formed as acounter electrode in the outer side of the dielectric layer and furtherthereon, an electrically conducting layer 5 such as electricallyconducting paste is preferably formed, thereby fabricating a capacitorbasic device. This device by itself or a stacked body resultant fromstacking these devices is connected with lead wires 6,7 and thereafter,the whole is completely molded with epoxy resin 8 or the like and thenput into use as a capacitor part in electric articles over a wide range.

[0005] In recent years, with the progress of digitization of electricalinstruments or high-speed processing of personal computers, a compactand large-capacitance capacitor or a capacitor showing low impedance inthe high frequency region is being demanded.

[0006] As the compact and large-capacitance capacitor, electrolyticcapacitors such as aluminum electrolytic capacitor and tantalumelectrolytic capacitor are known.

[0007] The aluminum electrolytic capacitor is advantageous in that alarge-capacitance capacitor can be obtained at a low cost but suffersfrom such problems that when an ion conducting liquid electrolyte isused as the electrolyte, the impedance in the high frequency region ishigh, the capacitance decreases accompanying the evaporation of theelectrolytic solution with the passing of time and the temperaturecharacteristics are inferior.

[0008] The tantalum electrolytic capacitor where a manganese oxide isgenerally used as the electrolyte, has such problems that the manganeseoxide predominantly produced by the thermal decomposition of manganesenitrate cannot be evaded from the possibility of the dielectric filmhaving damages at the thermal decomposition and due to the relativelyhigh specific resistance, the impedance in the high frequency region ishigh.

[0009] In order to solve these problems, it has been proposed to use anelectrically conducting polymer having electric conductivity as thesolid electrolyte. For example, use of an intrinsic conducting polymerhaving an electric conductivity of 10⁻³ to 10³ S/cm (see, JP-A-1-169914(the term “JP-A” as used herein means an “unexamined published Japanesepatent application”) (corresponding to U.S. Pat. No. 4,803,596)) and useof a polymer such as polyaniline (see, JP-A-61-239617), polypyrrole(see, JP-A-61-240625), a polythiophene derivative (see, JP-A-2-15611(corresponding to U.S. Pat. No. 4,910,645)) or polyisothianaphthene(see, JP-A-62-118511) are known. These electrically conducting polymerscomprising a π-conjugated structure are mostly used in the form of acomposition containing a dopant.

[0010] In recent years, not only the addition of a dopant but also acombination use with, for example, manganese dioxide (see, JP-B-6-101418(the term “JP-B” as used herein means an “examined Japanese patentpublication”) (corresponding to U.S. Pat. No. 4,959,753)) or filler(see, JP-A-9-320901) is employed.

[0011] With respect to the shape of the solid electrolyte, it has beenproposed to weld a metal onto an aluminum foil and thereby form astarting point for growing an electrically conducting polymer by theelectrolytic oxidative polymerization throughout the surface of thealuminum foil (see, JP-A-4-307917).

[0012] Also, a method of performing the alternate impregnation with amonomer solution and with an oxidizing agent solution each from 1 to 20times and the dipping in an oxidizing solution for 5 minutes to 5 hours,thereby improving the polymerization efficiency, has been proposed (see,JP-A-11-238648).

PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] These conventional methods for producing a solid electrolyticcapacitor using an electrically conducting polymer as the solidelectrolyte have the following problems.

[0014] (1) When monomer is used in forming a solid electrolyte layer, amixed solution of monomer and an oxidizing agent is used and the monomerin the mixed solution is polymerized by itself due to the oxidizingaction of the oxidizing agent in the mixed solution and converts intopolymer. This polymer must be discarded. Therefore, monomer in the mixedsolution cannot be used effectively and the use efficiency of thestarting material is very bad.

[0015] (2) The mixed solution of monomer and an oxidizing agent changesin the property and therefore, the process of forming the solidelectrolyte layer is unstable, for example, the oxidizing action of theoxidizing agent decreases to shorten the life of the mixed solution.

[0016] (3) In addition, even when the oxidizing agent solution and themonomer solution are separately prepared (two solutions) and the metalfoil substrate is alternately dipped in these solutions, the monomerdisadvantageously dissolves out into the oxidizing agent solution at thetime of dipping the substrate impregnated with the monomer solution inthe oxidizing agent solution for a predetermined time period, and themonomer polymerizes in the oxidizing agent solution. As a result, thelife of the oxidizing agent solution is extremely shortened. Also, inthe case of dipping the metal foil substrate in the oxidizing agentsolution and then in the monomer solution for a predetermined timeperiod, the life of the monomer solution is similarly shortened.

[0017] (4) In the preparation of the mixed solution of monomer and anoxidizing agent, the concentrations of and the mixing ratio betweenmonomer and the oxidizing agent have a certain limit, therefore, themonomer concentration cannot be freely selected and for forming thesolid electrolyte layer to have a desired thickness, for example, thenumber of times of performing the polymerization must bedisadvantageously increased.

[0018] (5) According to the method of repeating the alternate dipping inthe oxidizing agent solution and in the monomer solution to perform thepolymerization, a washing step is generally provided after eachpolymerization. This washing operation every each polymerization and thetime spent therefor not only lower the production efficiency of a solidelectrolytic capacitor device but also decrease the strength of thepolymer solid electrolyte layer part, because overlapping of the polymersolid electrolyte layers is reduced. Thus, improvement is necessary alsoin view of the capability of the solid electrolytic capacitor.

[0019] (6) Since the dielectric layer is formed only by anelectrochemical forming treatment in the later step, the cut surface(cut end part) formed in cutting the porous valve-acting metal into apredetermined shape is weak as compared with the part other than the cutsurface, and the solid electrolyte is liable to adhere thereto in asmall amount.

[0020] (7) In the masking part for insulating the anode part from thecathode part of the solid electrolytic capacitor and thereby preventingthe solid electrolyte from extending to the anode part, the solidelectrolyte is liable to fail in satisfactorily adhering and thecapacitance is disadvantageously reduced. Furthermore, the masking partworks out to a part of joining anodes in stacking the solid electrolyticcapacitor devices and therefore, this part is readily subject to astress, as a result, a large leakage current is generated and shortcircuit readily takes place.

[0021] (8) In the method of forming the solid electrolyte of anelectrically conducting polymer through the dipping in a solutioncontaining a monomer of organic polymer and in a solution containing anoxidizing agent, as described above, the amount of the solid electrolyteadhered is liable to be large in the center part of the substrate(aluminum foil) and small at the cut end part on the cut surface and atthe masking part. Furthermore, the balance in the adhering state betweenthe oxidizing agent and the monomer is readily lost-to fail in obtaininga polymer having constant performance.

DISCLOSURE OF THE INVENTION

[0022] As a result of extensive investigations for forming a solidelectrolyte at the cut end part and in the masking boundary region whiletaking account of the above-described problems, the present inventorshave found that the solid electrolyte can be formed in an increasedamount particularly on the cut surface and in the masking boundaryregion by gradually vaporizing the solvent of an oxidizing agentsolution on the substrate and polymerizing a monomer on the remainingoxidizing agent. The solid electrolytic capacitor obtained as such isverified to show improved adhesion between the dielectric layer and thesolid electrolyte formed on the dielectric film and to have excellentstability in various basic properties such as capacitance, dielectricloss (tan δ), leakage current and short circuit defective ratio, andalso in the reflow soldering heat resistance and the moisture resistanceload characteristics.

[0023] Furthermore, in the method where the solid electrolytic capacitorhaving the excellent properties as described above is produced bydipping the substrate into monomer solution of organic polymer ordispersant (hereinafter it is referred to simply as a solutioncontaining a monomer) and solution of an oxidizing agent or dispersant(hereinafter it is referred to simply as a solution containing anoxidizing agent), the present inventors have found that for reducing theelution of the monomer into the solution containing an oxidizing agentor the elution of the solution containing an oxidizing agent into thesolution containing a monomer, it is effective to control the dippingtime in the solution containing an oxidizing agent or in the solutioncontaining a monomer and also to control the polymerization time byadding a drying step at a predetermined temperature for a predeterminedtime after the dipping in the solution containing a monomer.Furthermore, it has been found that when the washing is performed at thefinal stage after the alternate dipping in the solution containing amonomer and in the solution containing an oxidizing agent, the dryingand the polymerization are repeated a predetermined number of times, theoverlapping between layers can be maintained and the solid electrolyteformed can have excellent properties.

[0024] The present invention provides a solid electrolytic capacitor anda method for producing the solid electrolytic capacitor as describedbelow.

[0025] 1. A solid electrolytic capacitor comprising a porousvalve-acting metal substrate having on the surface thereof a dielectricfilm and having provided on the dielectric film a solid electrolyte ofan electrically conducting polymer obtainable by oxidation-polymerizinga monomer of organic polymer using an oxidizing agent, wherein thethickness of the solid electrolyte layer in the peripheral part of thesubstrate is larger than the thickness of the solid electrolyte layer inthe center part of the substrate.

[0026] 2. A solid electrolytic capacitor comprising a porousvalve-acting metal substrate having on the surface thereof a dielectricfilm and having provided on the dielectric film a solid electrolyte ofan electrically conducting polymer obtainable by oxidation-polymerizinga monomer of organic polymer using an oxidizing agent, said valve-actingmetal porous substrate being cut into a predetermined shape, wherein thethickness of the solid electrolyte layer in the periphery of the cutsurface of the substrate is larger than the thickness of the solidelectrolyte layer in the center part of the substrate.

[0027] 3. A solid electrolytic capacitor comprising a porousvalve-acting metal substrate having on the surface thereof a dielectricfilm and having provided on the dielectric film a solid electrolyte ofan electrically conducting polymer obtainable by oxidation-polymerizinga monomer of organic polymer using an oxidizing agent, said valve-actingmetal porous substrate being cut into a predetermined shape, wherein thethickness of the solid electrolyte layer in the periphery of the cutsurface of the substrate and in the masking boundary part is larger thanthe thickness of the solid electrolyte layer in the center part of thesubstrate.

[0028] 4. The solid electrolytic capacitor as described in any one of 1to 3 above, wherein the partiality in the thickness of said solidelectrolyte is created by the solution chemical oxidative polymerizationor vapor phase chemical oxidative polymerization of a monomer of organicpolymer performed on said valve-acting metal substrate having thereon adielectric film.

[0029] 5. The solid electrolytic capacitor as described in 4 above,wherein the partiality in the thickness of said solid electrolyte iscreated by repeating an operation of alternately dipping saidvalve-acting metal substrate having thereon a dielectric film in asolution containing a monomer of organic polymer and in a solutioncontaining an oxidizing agent.

[0030] 6. The solid electrolytic capacitor as described in any one of 1to 5 above, wherein the porous valve-acting metal substrate has a plate-or foil-like shape.

[0031] 7. The solid electrolytic capacitor as described in 6 above,wherein the solid electrolyte is formed such that the center part of theporous valve-acting metal substrate has a guitar- or gourd-like crosssection in the longitudinal direction and in the transverse direction.

[0032] 8. The solid electrolytic capacitor as described in 7 above,wherein in the cross section of the center part of the substrate onwhich a solid electrolyte layer is formed, the difference between themaximum thickness and the minimum thickness is from 0 to 200 μm.

[0033] 9. The solid electrolytic capacitor as described in any one of 1to 8 above, wherein the porous valve-acting metal is a simple metalselected from aluminum, tantalum, niobium and titanium, or an alloythereof.

[0034] 10. The solid electrolytic capacitor as described in any one of 1to 9 above, wherein the monomer of organic polymer for forming theelectrically conducting polymer is a compound containing a 5-memberheterocyclic ring, or a compound having an aniline skeleton.

[0035] 11. The solid electrolytic capacitor as described in 10 above,wherein the compound containing a 5-member heterocyclic ring is acompound having a thiophene skeleton or a polycyclic sulfide skeleton.

[0036] 12 The solid electrolytic capacitor as described in 11 above,wherein the monomer compound having a thiophene skeleton is3-ethylthiophene, 3-hexylthiophene, 3,4-dimethylthiophene,3,4-methylenedioxythiophene, and 3,4-ethylenedioxythiophene.

[0037] 13. A solid electrolytic multilayer capacitor obtainable bystacking a plurality of sheets of the solid electrolytic capacitor asdescribed in 1 to 12 above.

[0038] 14. A method for producing a solid electrolytic capacitor,comprising providing a solid electrolyte of an electrically conductingpolymer on a dielectric film on the surface of a porous valve-actingmetal substrate by the oxidative polymerization of a monomer of organicpolymer using an oxidizing agent, wherein the solid electrolyte layer isformed such that the thickness in the peripheral part of the substrateis larger than the thickness in the center part of the substrate.

[0039] 15. The method for producing a solid electrolytic capacitor asdescribed in 14 above, wherein the partiality in the thickness of saidsolid electrolyte is created by the solution chemical oxidativepolymerization or vapor phase chemical oxidative polymerization of amonomer of organic polymer performed on said valve-acting metalsubstrate having thereon a dielectric film.

[0040] 16. The method for producing a solid electrolytic capacitor asdescribed in 15 above, wherein the partiality of the thickness of saidsolid electrolyte is created by repeating an operation of alternatelydipping said valve-acting metal substrate having thereon a dielectricfilm in a solution containing a monomer of organic polymer and in asolution containing an oxidizing agent.

[0041] 17. The method for producing a solid electrolytic capacitor asdescribed in 16 above, wherein the substrate is alternately dipped in aeach solution for less than 5 minutes.

[0042] 18. The method for producing a solid electrolytic capacitor asdescribed in 16 above, wherein the alternate dipping is repeated from 15to 30 times.

[0043] 19. The method for producing a solid electrolytic capacitor asdescribed in 16 above, wherein said valve acting metal substrate is leftstanding in air for from 5 seconds to 15 minutes after the dipping inthe solution containing a monomer.

[0044] 20. The method for producing a solid electrolytic capacitor asdescribed in 16 above, wherein said valve acting metal substrate is leftstanding in air for from 10 seconds to 15 minutes after the dipping insaid solution containing an oxidizing agent.

[0045] 21. The method for producing a solid electrolytic capacitor asdescribed in 19 or 20 above, wherein said valve acting metal substrateis left standing in air at a temperature of 0 to 60° C.

[0046] 22. The method for producing a solid electrolytic capacitor asdescribed in 16 above, wherein after the step of dipping said valveacting metal substrate alternately in the solution containing a monomerand in the solution containing an oxidizing agent to perform thepolymerization, said valve acting metal substrate is washed.

[0047] 23. A method for producing a solid electrolytic capacitor,comprising forming a solid electrolyte layer of an electricallyconducting polymer on the surface of a valve acting metal substratehaving thereon a dielectric film porous body, using a solutioncontaining a monomer capable of forming an electrically conductingpolymer under the action of an oxidizing agent, and a solutioncontaining an oxidizing agent, which has a step of dipping said valveacting metal substrate alternately in the solution containing a monomerand in the solution containing an oxidizing agent each for less than 5minutes to perform the polymerization.

[0048] 24. A method for producing a solid electrolytic capacitor,comprising forming a solid electrolyte layer of an electricallyconducting polymer on the surface of a valve acting metal substratehaving thereon a dielectric film porous body, using a solutioncontaining a monomer capable of forming an electrically conductingpolymer under the action of an oxidizing agent, and a solutioncontaining an oxidizing agent, wherein a step of dipping said valveacting metal substrate alternately in the solution containing a monomerand in the solution containing an oxidizing agent each for less than 5minutes is repeated from 15 to 30 times to perform the polymerization.

[0049] 25. The method for producing a solid electrolytic capacitor asdescribed in 23 or 24 above, wherein said valve acting metal substrateis left standing in air for from 5 seconds to 15 minutes after thedipping in the solution containing a monomer.

[0050] 26. The method for producing a solid electrolytic capacitor asdescribed in any one of 23 to 25 above, wherein said valve acting metalsubstrate is left standing in air for from 10 seconds to 15 minutesafter the dipping in said solution containing an oxidizing agent.

[0051] 27. The method for producing a solid electrolytic capacitor asdescribed in 25 or 26 above, wherein said valve acting metal substrateis left standing in air at a temperature of 0 to 60° C.

[0052] 28. The method for producing a solid electrolytic capacitor asdescribed in any one of 23 to 27 above, wherein after repeating the stepof performing polymerization, said valve acting metal substrate iswashed.

[0053] 29. The method for producing a solid electrolytic capacitor asdescribed in any one of 23 to 28 above, wherein a part of the solidelectrolyte layer formed of an electrically conducting polymer has alamellar structure or a fibril structure.

[0054] 30. The method for producing a solid electrolytic capacitor asdescribed in any one of 23 to 29 above, wherein the monomer for formingan electrically conducting polymer is a compound containing aheterocyclic 5-membered ring.

[0055] 31. The method for producing a solid electrolytic capacitor asdescribed in any one of 23 to 30 above, wherein the monomer for formingan electrically conducting polymer is a compound having an anilineskeleton.

[0056] 32. The method for producing a solid electrolytic capacitor asdescribed in 30 above, wherein the compound containing a heterocyclic5-membered ring has a thiophene skeleton.

[0057] 33. The method for producing a solid electrolytic capacitor asdescribed in 30 above, wherein the monomer for forming an electricallyconducting polymer is selected from the group consisting of3-ethylthiophene, 3-hexylthiophene, 3,4-dimethylthiophene,3,4-methylenedioxythiophene, 3,4-ethylenedioxythiophene,1,3-dihydroisothianaphthene, and 3,4-ethylenedioxyfurane.

[0058] 34. The method for producing a solid electrolytic capacitor asdescribed in any of 23 to 33 above, wherein said valve acting metal is asingle metal selected from aluminum, tantalum, niobium and titanium, oran alloy thereof.

[0059] 35. A solid electrolytic capacitor obtained by the productionmethod described in 23 above.

[0060] 36. A solid electrolytic capacitor obtained by the productionmethod described in 24 above.

[0061] 37. A solid electrolytic capacitor obtained by the productionmethod described in any one of 25 to 34 above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062]FIG. 1 is a cross section of a capacitor device.

[0063]FIG. 2 is a schematic longitudinal cross section of a capacitordevice material of Example 1.

[0064]FIG. 3 is a cross section of a solid electrolytic capacitorobtained by stacking the capacitor devices of Example 1 .

[0065]FIG. 4 is a schematic longitudinal cross section of a capacitordevice material of Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0066] The dielectric film on the surface of a substrate for use in thepresent invention is usually formed by subjecting a porous metal moldedmaterial having valve action to an electrochemical forming treatment orthe like.

[0067] The forming solution and the conditions (e.g., forming voltage)for use in the electrochemical forming treatment are previously verifiedand optimally decided according to the capacitance, voltage resistanceand the like required for the solid electrolytic capacitor produced. Inperforming the electrochemical forming treatment, a masking is generallyprovided for preventing the forming solution from bleeding up to theportion which works out to the anode of the solid electrolytic capacitorand at the same time, for ensuring the insulation of the anode from thesolid electrolyte (cathode part) which is formed in the later step.

[0068] The masking material is not limited and a general heat-resistantresin may be used and a composition comprising a solvent-soluble orswellable heat-resistant resin or a precursor thereof, an inorganicpowder and a cellulose-based resin (see, JP-A-11-80596) is preferablyused. Specific examples thereof include polyphenylsulfone (PPS),polyethersulfone (PES), cyanic ester resin, fluororesin (e.g.,tetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ethercopolymer), low molecular polyimide, and derivatives and precursorsthereof. Among these, preferred are low molecular polyimide,polyethersulfone, fluororesin, and precursors thereof.

[0069] In general, for forming an electrically conducting polymer on anoxide dielectric film, a method of forming an electrically conductivepolymer layer by vapor phase polymerization or electrolyticpolymerization (see, JP-A-3-6217), a solution chemical polymerization ofadhering a monomer of organic polymer on an oxide dielectric film andpolymerizing the monomer in an oxidizing agent solution (see,JP-A-11-251191), and an electrolytic polymerization method of changingthe feeding point to the anode body at predetermined intervals using aswitching device and thereby equalizing the thickness of theelectrically conducting polymer layer (see, JP-A-11-283878) may be used.In the present invention, a method of subjecting a monomer of organicpolymer to a solution chemical oxidative polymerization treatmentcontaining a step of dipping a porous valve-acting metal substrate in anoxidizing solution and drying it to gradually increase the concentrationof the oxidizing agent solution on the substrate, and a vapor phasechemical oxidative polymerization are preferably used. Among these, thesolution chemical oxidative polymerization method is more preferred.

[0070] In producing the solid electrolyte by the vapor phase chemicaloxidative polymerization which is one example of the present invention,the partial pressure of a monomer (in the case of introducing themonomer as a mixed gas) and the pressure (in the case of introducing thegas under controlled pressure) vary depending on the kind of thesubstituent of the compound or the kind of the solvent or the like,however, the pressure is generally preferably from 10⁻³ to 10 atm, morepreferably from 10⁻² to 5 atm.

[0071] The reaction temperature at the vapor phase chemical oxidativepolymerization is decided by the kind and concentration of each compoundhaving the polymerization initiating ability and the partial pressure(pressure) of the polymerizable monomer and is not particularly limited,however, the reaction temperature is generally from −70 to 250° C.,preferably from 0 to 150° C., more preferably from 15 to 100° C.

[0072] According to the present invention, as described in Exampleslater, an aluminum foil having thereon an oxide dielectric film isdipped, for example, in an isopropyl alcohol (IPA) solution of3,4-ethylenedioxythiophene (EDT), air-dried to mostly remove the IPA,dipped in an aqueous solution containing about 20% by mass of anoxidizing agent (ammonium persulfate) and then heated at about 40° C.for 10 minutes, or this procedure is repeated, whereby a polymer ofpoly(3,4-ethylenedioxythiophene) can be obtained in such a state thatthe thickness of the solid electrolyte layer is larger in the peripheryof the substrate than in the center part.

[0073] The reason why this partiality in the thickness of the solidelectrolyte layer is created, is not elucidated in detail, however, itis presumed that the condition under heating at about 40° C. provides anenvironment of readily causing dissipation of the moisture on thedielectric film having a large surface area, the oxidizing agentsolution moves to the cut surface (cut end part) of the substrate and tothe masking part due to capillarity, water gradually evaporates therefrom the oxidizing agent solution to give a high concentration, and amonomer contacts with this and polymerizes to allow the polymer toadhere to the cut end part and in the vicinity of the masking boundaryregion in a large amount, as a result, the thickness of the solidelectrolyte in the periphery of the substrate becomes larger than thethickness of the solid electrolyte layer in the center part of thesubstrate.

[0074] In other words, the main factor of forming a solid electrolytehaving a guitar- or gourd-like cross section in the longitudinaldirection and in the transverse direction at the center part of theporous valve-acting metal substrate, which is a characteristic featureof the solid electrolytic capacitor of the present invention, is themode of forming a polymer, where the oxidizing solution is concentratedto a high concentration on the cut surface (cut end part) and in themasking part due to the relation between the surface tension and theosmotic pressure of the formed porous body and the polymer is allowed todeposit there. This polymer formation mode at the edge part seems tolead to the success in gaining preferred properties, namely, reductionin the short circuit defective ratio of solid electrolytic capacitorsand in the leakage current.

[0075] The partiality in the thickness of the solid electrolyte layervaries depending on the conditions in forming the solid electrolyte,such as the repetition number of an operation of dipping thevalve-acting metal substrate in a solution containing a monomer oforganic polymer and in a solution containing an oxidizing agent,however, for obtaining preferred solid electrolytic capacitorcharacteristics, the difference between the maximum thickness and theminimum thickness is from 0 to 200 μm, preferably from 0 to 180 μm, morepreferably from 0 to 150 μm, in the cross section of the center part ofthe substrate on which a solid electrolyte layer is formed.

[0076] The solid electrolyte layer of an electrically conducting polymerobtained by the method of the present invention has a fibril structureor a lamella (thin layer-like) structure. These structures havewidespread overlapping among the polymer chains that enables electrontransfer between polymers and this is considered to contribute to theimprovement in the electric conductivity and in the properties such aslow impedance.

[0077] According to the solution chemical polymerization method, amonomer is attached onto a dielectric film having microfine pores of ananode substrate, oxidative polymerization thereof is induced by theaction of an oxidizing agent and water in air in the presence of acompound which can work out to a dopant of the electrically conductingpolymer, and the polymer composition produced on the dielectric surfaceforms a solid electrolyte. At this time, for forming a good polymercomposition, each dipping time in (namely, each impregnation time with)the solution containing a monomer and in the solution containing anoxidizing agent must be adjusted to control the amount of the monomerattached and the amount of the oxidizing agent attached. For example, ifthe dipping time is too long, the polymerization reaction cannot becompleted and the polymer composition obtained is liable to have a lowmolecular weight. If the dipping time in the solution containing anoxidizing agent having an unsaturated concentration is too long, theoxidizing agent attached to the metal foil substrate during the passingthrough the previous steps including the drying step re-dissolves and atthe same time, the monomer attached or the polymer produced is alsoeluted or flows out, as a result, not only the producing of the polymeris retarded but also the solution containing an oxidizing agent iscontaminated with the effluent. The same may occur in the case of thedipping in the solution containing a monomer.

[0078] With respect to the phenomenon brought about, for example,coloration of the solution containing an oxidizing agent or solutioncontaining a monomer due to low molecular weight components, suspensionof the polymerized matter, liability toward the reduction in the weightof the adhered and formed solid electrolyte, and change in the viscosityor specific gravity of the solution containing a monomer may occur.

[0079] Accordingly, in the method of the present invention, the timespent for each dipping in the solution containing a monomer and in thesolution containing an oxidizing agent is specified to be from a timesufficiently long to allow the monomer component or the oxidizing agentcomponent in the solution containing the component to adhere to thedielectric surface of the metal foil substrate to a time less than 5minutes, preferably from 0.1 seconds to 2 minutes, more preferably from1 second to 1 minute.

[0080] After the impregnation with the solution containing a monomer,the substrate must be left standing in air for a predetermined time tovaporize the solvent and thereby uniformly attach the monomer onto thedielectric surface and also onto the polymer composition. The conditionsfor this standing vary depending on the kind of solvent. Generally, thestanding temperature is from about 0° C. to the boiling point of thesolvent and the standing time is approximately from 5 seconds to 15minutes. For example, in the case of an alcohol-type solvent, standingof 5 minutes or less may be sufficient. By this standing, the monomercan uniformly adhere to the dielectric surface and at the subsequentdipping in the solution containing an oxidizing agent, the contaminationmay be reduced.

[0081] After the dipping in the solution containing a monomer and in thesolution containing an oxidizing agent, the metal foil substrate is keptin air at a constant temperature for a predetermined time tooxidation-polymerize the monomer.

[0082] The polymerization temperature varies depending on the kind ofthe monomer, however, for example, in the case of pyrrole, thepolymerization temperature is 5° C. or less and in the case of athiophene-type monomer, it is from about 30° C. to about 60° C.

[0083] The polymerization time depends on the amount of the monomerattached at the dipping. The amount of the monomer attached variesdepending on the concentration or viscosity of each solution containingthe monomer or oxidizing agent and cannot be indiscriminately specified.However, in general, when the amount of the monomer attached per once isreduced, the polymerization time can be shortened, whereas when theamount of the monomer attached per once is increased, the polymerizationtakes a long time.

[0084] In the method of the present invention, the polymerization timeper once is from 10 seconds to 15 minutes, preferably from 3 to 10minutes.

[0085] The electrically conducting polymer layer formed directly on thedielectric film according to the method of the present invention isconfirmed to have a lamella structure or a fibril structure on thephotograph taken through an electron microscope.

[0086] The lamella structure and the fibril structure of theelectrically conducting polymer are considered to contribute to theimprovement of the one-dimensional property of the polymer chain, whichis one of the factors of increasing the electric conductivity, and alsoto the widespread overlapping among the polymer chains. Furthermore,these structures seem to have an advantageous effect on the increase ofelectrical conductivity of the polymer solid electrolyte and on theimprovement of the capacitor properties, such as reduction of theimpedance.

[0087] In the method of the present invention, the dipping frequencymust be controlled so that the electrically conducting polymer compoundcan be formed to have a thickness large enough to ensure the resistanceagainst moisture, heat, stress and the like. A desired solid electrolytelayer may be easily formed by repeating the above-described productionprocess 15 times or more, preferably from 20 to 30 times, per one anodesubstrate.

[0088] The step for forming a solid electrolyte for use in a solidelectrolytic capacitor is a step of alternately dipping the anode bodyobtained by forming a dielectric film on a valve acting metal in asolution containing a monomer and in a solution containing an oxidizingagent to repeatedly and alternately attach the monomer and the oxidizingagent to the anode body, and allowing the chemical oxidationpolymerization to proceed in air.

[0089] At this time, the chemical oxidation polymerization for formingan electrically conducting polymer layer containing as a repeating unitthe monomer or a derivative thereof is performed by setting the humiditycondition in the atmosphere to from 10% to less than 80%, preferablyfrom 15% to less than 60%, more preferably from 20% to less than 50%.

[0090] The temperature and the pressure in the atmosphere vary dependingon the kind and the polymerization method of the polymer composition andcannot be indiscriminately specified, however, in general, thetemperature is preferably in the range from −70° C. to 250° C. and thepressure is preferably an atmospheric pressure or lower.

[0091] The concentration of the solution containing a monomer is from 3to 50 mass %, preferably from 5 to 35 mass %, more preferably from 10 to25 mass %. The concentration of the solution containing an oxidizingagent is from 5 to 70 mass %, preferably from 15 to 50 mass %. Thesolution containing a monomer and the solution containing an oxidizingagent each has a viscosity of 100 cP (centipoise) or less, preferably 30cP or less, more preferably from 0.6 to 10 cP.

[0092] According to the present invention, a solid electrolyte formed ofan electrically conducting polymer having a layer structure (lamellarstructure or fibril structure) can be obtained by the alternate dippingin the solution containing a monomer and in the solution containing anoxidizing agent. However, in order to improve the one-dimensionalproperty of the polymer chain in the layer and generate overlappingamong the polymer chains, it has been found preferred not to perform thewashing every each polymerization but perform the washing at the finalstage. By doing so, the excess (unreacted) monomer remaining unreactedin the polymerization step can be polymerized in the next step, as aresult, a solid electrolyte comprising an electrically conductingpolymer having a layer structure favored with widespread overlapping canbe formed.

[0093] In the solid electrolyte having a lamellar structure or a fibrilstructure, which is obtained by the method of the present invention andcovering the outer surface of the anode body, relatively continuous orindependent spaces are generated and this space has a function torelieve the effect of thermal stress, mechanical stress and the likeimposed during the production process of a capacitor, such as molding.This can be said to be a useful structure capable of coping with notonly the stresses imposed during the production process but also variousstresses coming down from the environment where the capacitor isactually used.

[0094] In one preferred embodiment of the present invention, the processfor forming a solid electrolyte includes a step of dipping a valveacting metal anode foil having formed thereon the above-describeddielectric film layer in a solution containing an oxidizing agent(Solution 1) and a step of dipping it in a solution containing a monomerand a dopant (Solution 2). With respect to the order of dippingoperations, an order of dipping the valve acting metal anode foil inSolution 1 and then dipping it in Solution 2 (regular order) may be usedor an order reversed thereto of dipping the valve acting metal anodefoil in Solution 2 and then dipping it in Solution 1 may also be used.

[0095] In another embodiment, the process may include a step of dippingthe anode foil in a solution containing an oxidizing agent and a dopant(Solution 3) and a step of dipping it in a solution containing a monomer(Solution 4). Also in this case, an order of dipping the anode foil inSolution 3 and then dipping it in Solution 4 (regular order) or an orderreversed thereto of dipping the anode foil in Solution 4 and thendipping it in Solution 3 may be used. Solutions 1 to 4 each may be usedin the state of suspension. Furthermore, the dipping may be replaced bythe coating.

[0096] The solvents in Solutions 1 to 4 may be the same or differentsolvent systems may be used, if desired. Depending on the kind ofsolvent, a drying step may be separately interposed between Solution 1and Solution 2 or between Solution 3 and Solution 4. Furthermore,washing with a solvent may be performed after the formation of the solidelectrolyte.

[0097] The metal having valve action, which can be used in the presentinvention, is a simple metal such as aluminum, tantalum, niobium,titanium, zirconium, magnesium and silicon, or an alloy thereof. Themetal may have any shape as long as it is in the form of a porous moldedmaterial such as an etched product of rolled foil or a sintered body offine powder.

[0098] For the anode substrate, a porous sintered body of theabove-described metal, a plate (including ribbon, foil and the like)surface-treated, for example, by etching, a wire and the like may beused, however, a plate and a foil are preferred. On the surface of thismetal porous body, an oxide dielectric film is formed and a known methodmay be used therefor. For example, in the case of using a sintered bodyof tantalum powder, the oxide film may be formed on the sintered body bythe anodization in an aqueous phosphoric acid solution.

[0099] The thickness of the valve-acting metal foil varies depending onthe use end, however, in general, a foil having a thickness of about 40to about 150 μm is used. The size and the shape of the valve-actingmetal foil also vary depending on the use end, however, the metal foilas a plate-like device unit preferably has a rectangular form having awidth of about 1 to about 50 mm and a length of about 1 to about 50 mm,more preferably a width of about 2 to about 15 mm and a length of about2 to about 25 mm.

[0100] Examples of the aqueous solution-type oxidizing agent which canbe used for the formation of the solid electrolyte in the presentinvention include peroxodisulfuric acid and Na, K and NH₄ salts thereof,cerium(IV) nitrate, ammonium cerium(IV) nitrate, iron(III) sulfate,iron(III) nitrate and iron(III) chloride. Examples of the organicsolvent-type oxidizing agent include ferric salts of an organic sulfonicacid, such as iron(III) dodecylbenzenesulfonate and iron(III)p-toluenesulfonate. Examples of the organic solvent used here includeγ-butyrolactone and monohydric alcohols such as butanol and isopropanol.The concentration of the oxidizing agent solution is preferably from 5to 50% by mass and the temperature of the oxidizing agent solution ispreferably from −15 to 60° C.

[0101] The electrically conducting polymer constituting the solidelectrolyte for use in the present invention is a polymer of an organichigh molecular monomer having a π electron-conjugate structure and thepolymerization degree thereof is from 2 to 2,000, more preferably from 5to 1,000. Specific examples thereof include electrically conductingpolymers containing as a repeating unit a structure shown by a compoundhaving a thiophene skeleton, a compound having a polycyclic sulfideskeleton, a compound having a pyrrole skeleton, a compound having afuran skeleton or a compound having an aniline skeleton, however, theelectrically conducting polymer is not limited thereto.

[0102] With respect to the monomer compound used as a starting materialof the electrically conducting polymer, examples of the compound havinga thiophene skeleton include derivatives such as 3-methylthiophene,3-ethylthiophene, 3-propylthiophene, 3-butylthiophene,3-pentylthiophene, 3-hexylthiophene, 3-heptylthiophene,3-octylthiophene, 3-nonylthiophene, 3-decylthiophene, 3-fluorothiophene,3-chlorothiophene, 3-bromothiophene, 3-cyanothiophene,3,4-dimethylthiophene, 3,4-diethylthiophene, 3,4-butylenethiophene,3,4-methylenedioxythiophene and 3,4-ethylenedioxythiophene. Thesecompounds may be a compound generally available on the market or may beprepared by a known method (a method described, for example, inSynthetic Metals, Vol. 15, page 169 (1986)), however, the presentinvention is not limited thereto.

[0103] Specific examples of the monomer compound having a polycyclicsulfide skeleton include compounds having a 1,3-dihydro-polycyclicsulfide (also called 1,3-dihydrobenzo-[c]thiophene) skeleton andcompounds having a 1,3-dihydronaphtho[2,3-c]thiophene skeleton.Furthermore, compounds having a 1,3-dihydroanthra[2,3-c]thiopheneskeleton and compounds having a 1,3-dihydronaphthaceno[2,3-c]thiopheneskeleton may be used. These compounds may be prepared by a known method,for example, by the method described in JP-A-8-3156.

[0104] In addition, compounds having a1,3-dihydronaphtho[1,2-c]thiophene skeleton such as1,3-dihydrophenanthra[2,3-c]thiophene derivatives, and compounds havinga 1,3-dihydrotriphenylo[2,3-c]thiophene skeleton such as1,3-dihydrobenzo[a]anthraceno[7,8-c]thiophene derivatives, may also beused.

[0105] A compound arbitrarily containing nitrogen or N-oxide in thecondensed ring may also be used and examples thereof include1,3-dihydrothieno[3,4-b]quinoxaline,1,3-dihydrothieno[3,4-b]quinoxaline-4-oxide and1,3-dihydrothieno[3,4-b]quinoxaline-4,9-dioxide, however, the presentinvention is not limited thereto.

[0106] Examples of the monomer compound having a pyrrole skeletoninclude derivatives such as 3-methylpyrrole, 3-ethylpyrrole,3-propylpyrrole, 3-butylpyrrole, 3-pentylpyrrole, 3-hexylpyrrole,3-heptylpyrrole, 3-octylpyrole, 3-nonylpyrrole, 3-decylpyrrole,3-fluoropyrrole, 3-chloropyrrole, 3-bromopyrrole, 3-cyanopyrrole,3,4-dimethylpyrrole, 3,4-diethylpyrrole, 3,4-butylenepyrrole,3,4-methylenedioxypyrrole and 3,4-ethylenedioxypyrrole. These compoundsmay be a commercially available compound or may be prepared by a knownmethod, however the present invention is not limited thereto.

[0107] Examples of the compound having a furan skeleton includederivatives such as 3-methylfuran, 3-ethylfuran, 3-propylfuran,3-butylfuran, 3-pentylfuran, 3-hexylfuran, 3-heptylfuran, 3-octylfuran,3-nonylfuran, 3-decylfuran, 3-fluorofuran, 3-chlorofuran, 3-bromofuran,3-cyanofuran, 3,4-dimethylfuran, 3,4-diethylfuran, 3,4-butylenefuran,3,4-methylenedioxyfuran and 3,4-ethylenedioxyfuran. These compounds maybe a commercially available compound or may be prepared by a knownmethod, however the present invention is not limited thereto.

[0108] Examples of the compound having an aniline skeleton includederivatives such as 2-methylaniline, 2-ethylaniline, 2-propylaniline,2-butylaniline, 2-pentylaniline, 2-hexylaniline, 2-heptylaniline,2-octylaniline, 2-nonylanilin, 2-decylaniline, 2-fluoroaniline,2-chloroaniline, 2-bromoaniline, 2-cyanoaniline, 2,5-dimethylaniline,2,5-diethylaniline, 2,3-butyleneaniline, 2,3-methylenedioxyaniline and2,3-ethylenedioxyaniline. These compounds may be a commerciallyavailable product or may be prepared by a known method, however, thepresent invention is not limited thereto.

[0109] Among these, the compounds having a thiophene skeleton or apolycyclic sulfide skeleton are preferred, and 3,4-ethylenedioxythiphene(EDT) and 1,3-dihydroisothianaphthene are more preferred.

[0110] The solvent for the monomer of organic polymer is preferably amonohydric alcohol (e.g., methyl alcohol, ethyl alcohol, n-propylalcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol,tert-butyl alcohol). The monomer solution may have any monomerconcentration without limitation.

[0111] The conditions for the polymerization of the compound selectedfrom the group consisting of the above-described compounds are notparticularly limited and the polymerization may be easily performedafter previously confirming the preferred conditions by a simple test.

[0112] The compounds selected from the group consisting of theabove-described compounds may also be used in combination and the solidelectrolyte may be formed as a copolymer. In this case, the compositionratio and the like of polymerizable monomers depend on thepolymerization conditions and the preferred composition ratio andpolymerization conditions may be confirmed by a simple test.

[0113] For example, a method where an EDT monomer and an oxidizing agenteach preferably in the form of a solution are coated separately oneafter another or coated simultaneously on an oxide film layer of a metalfoil to form a solid electrolyte (see, JP-A-2-15611 and JP-A-10-32145),may be used.

[0114] 3,4-Ethylenedioxythiophene (EDT) which is preferably used in thepresent invention is well soluble in the above-described monohydricalcohol but low in the affinity for water, therefore, on contacting withan aqueous oxidizing agent solution of high concentration, thepolymerization of EDT aggressively proceeds on the interface thereof andan electrically conducting polymer solid electrolyte layer having afibril structure or a lamella (thin layer-like) structure is formed.

[0115] Examples of the solvent for the solutions used in the productionmethod of the present invention and the solvent for washing after theformation of the solid electrolyte include ethers such astetrahydrofuran (THF), dioxane and diethylether; ketones such as acetoneand methyl ethyl ketone; aprotic polar solvents such asdimethylformamide, acetonitrile, benzonitrile, N-methylpyrrolidone (NMP)and dimethylsulfoxide (DMSO); esters such as ethyl acetate and butylacetate; non-aromatic chlorine-based solvents such as chloroform andmethylene chloride; nitro compounds such as nitromethane, nitroethaneand nitrobenzene; alcohols such as methanol, ethanol and propanol;organic acids such as formic acid, acetic acid and propionic acid; acidanhydrides of the organic acid (e.g., acetic anhydride); water; andmixed solvents thereof. Among these, preferred are water, alcohols,ketones and mixed systems thereof.

[0116] The electrically conducting polymer for use in the presentinvention uses an arylsulfonic acid-based dopant. Examples of thestarting material for dopant, which can be used, include salts ofbenzenesulfonic acid, toluenesulfonic acid, naphthalenesulfonic acid,anthracenesulfonic acid, anthraquionenesulfonic acid and the like.

[0117] The thus-produced solid electrolyte has an electric conductivityof about 0.1 to about 200 S/cm, preferably from about 1 to about 150S/cm, more preferably from about 10 to about 100 S/cm.

[0118] On the electrically conducting polymer composition layer formed,an electrically conducting layer is preferably provided so as to attaingood electric contact with the cathode lead. For example, anelectrically conducting paste is solidified, a metal is plated ordeposited, or an electrically conducting resin film is formed.

[0119] Usually, multiple sheets of the thus-obtained solid electrolyticcapacitor devices are stacked to have a desired capacitance and afterconnecting a lead, the stacked body is applied with a jacket using resinmolding, a resin case, a metal-made jacket case or resin dipping,thereby completing a capacitor product for various uses.

BEST MODE FOR CARRYING OUT THE INVENTION

[0120] The present invention is described in detail below by referringto the Examples, however, the present invention should not be construedas being limited thereto.

EXAMPLE 1

[0121] An etched aluminum foil was cut into a size of 3 mm×10 mm and apolyimide solution was circumferentially coated on both surfaces in awidth of 1 mm to divide the surface into a 4-mm portion and a 5-mmportion in the long axis direction, and then dried to form a masking.The 3 mm×4 mm portion of this etched aluminum forming foil waselectrochemically formed with an aqueous 10% by mass ammonium adipatesolution by applying a voltage of 13 V, as a result, an oxide dielectricfilm was formed at the cut end part. Thereafter, this 3 mm×4 mm portionof the aluminum foil was dipped in 1.2 mol/L of an isopropyl alcohol(IPA) solution having dissolved therein 5 g of3,4-ethylenedioxythiophene (produced by Bayer AG) for 5 seconds, driedat room temperature for 5 minutes, and then dipped in 2 mol/L of anaqueous ammonium persulfate solution having suspended therein sodiumanthraquinone-2-sulfonate to a concentration of 0.07% by mass, for 5seconds. Subsequently, this aluminum foil was left standing in anatmosphere at 40° C. for 10 minutes to allow the oxidationpolymerization to proceed. By repeating the dipping step and thepolymerization step 25 times in total, a solid electrolyte layer of anelectrically conducting polymer was formed on the outer surface of theetched aluminum foil. The finally producedpoly(3,4-ethylenedioxythiophene) was washed with warm water at 50° C.and then dried at 100° C. for 30 minutes to complete the formation ofthe solid electrolyte layer.

[0122] Using a thicknessmeter (manufactured by Peacock Corp.:Digitaldial Gage DG-205 (accuracy of 3 μm)), the thickness was measuredby slowly putting the aluminum foil into the measuring part of thethicknessmeter.

[0123] As a result, the thickness (h₁) of the overhanging portion at theperipheral part shown in the schematic view of FIG. 2 was 260 μm, thethickness (h₂) of the constriction portion at the center part was 210μm, and the difference (h₁−h₂) in the film thickness was 50 μm.

[0124] Then, carbon paste and silver paste were applied to the aluminumfoil in the portion where the electrically conducting polymercomposition layer was formed. Four sheets of the thus-prepared aluminumfoils were stacked and a cathode lead was connected thereto. To theportion where the electrically conducting polymer was not formed, ananode lead was connected by welding. The resulting device was moldedwith an epoxy resin and aged at 125° C. for 2 hours by applying theretoa rated voltage. In total, 30 units of capacitors were fabricated.

[0125] These 30 units of capacitors were measured on the capacitance andthe loss factor (tan δ×100%) at 120 Hz as initial properties and alsomeasured on the impedance at a resonance frequency and the leakagecurrent. The leakage current was measured one minute after the ratedvoltage was applied. In Table 1, respective averages of these measuredvalues, the defective ratio when a device having a leakage current of0.59 μA (0.002 CV) or more is evaluated as a defective unit, and thenumber of short-circuited products are shown. The average of the leakagecurrent values is a value calculated exclusive of defectives. In Table2, the results in the reflow soldering test and the subsequent moistureresistance test are shown. In the moisture resistance test, a devicehaving a leakage current of 11.8 μA (0.04 CV) or more was evaluated as adefective. The reflow soldering test was performed by passing the devicethrough a temperature zone of 230° C. for 30 seconds and the moistureresistance test was performed by allowing the device to stand inhigh-temperature and high-humidity conditions of 85° C. and 85% RH for240 hours.

EXAMPLE 2

[0126] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for using iron(III) sulfate in place of ammoniumpersulfate and using dihydroisothianaphthene in place of3,4-ethylenedioxythiophene in Example 1.

[0127] The thickness of the overhanging part of the solid electrolytelayer was measured in the same manner as in Example 1 and found to be250 μm, the thickness of the constriction portion at the center part was200 μm, and the difference (h₁−h₂) in the film thickness was 50 μm.

[0128] These capacitor devices were evaluated on the properties in thesame manner as in Example 1. The results obtained are shown in Tables 1and 2.

EXAMPLE 3

[0129] 30 Units of capacitors were fabricated in the same manner as inExample 1 except that pyrrole was used in place of3,4-ethylenedioxythiophene in Example 1 and at this time, the pyrrolesolution impregnated was dried at 3° C. for 5 minutes and thereafter, anoxidizing agent solution was impregnated to perform the polymerizationat 5° C. for 10 minutes.

[0130] The thickness of the overhanging part of the solid electrolytelayer was measured in the same manner as in Example 1 and found to be280 μm, the thickness of the constriction portion at the center part was210 μm, and the difference (h₁−h₂) in the film thickness was 70 μm.

[0131] These capacitor devices were evaluated on the properties in thesame manner as in Example 1. The results obtained are shown in Tables 1and 2.

EXAMPLE 4

[0132] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for using furan in place of 3,4-ethylenedioxythiophenein Example 1.

[0133] The thickness of the overhanging part of the solid electrolytelayer was measured in the same manner as in Example 1 and found to be260 μm, the thickness of the constriction portion at the center part was200 μm, and the difference (h₁−h₂) in the film thickness was 60 μm.

[0134] These capacitor devices were evaluated on the properties in thesame manner as in Example 1. The results obtained are shown in Tables 1and 2.

EXAMPLE 5

[0135] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for using aniline in place of3,4-ethylenedioxythiophene in Example 1.

[0136] The thickness of the overhanging part of the solid electrolytelayer was measured in the same manner as in Example 1 and found to be270 μm, the thickness of the constriction portion at the center part was210 μm, and the difference (h₁−h₂) in the film thickness was 60 μm.

[0137] These capacitor devices were evaluated on the properties in thesame manner as in Example 1. The results obtained are shown in Tables 1and 2.

COMPARATIVE EXAMPLE 1

[0138] 3,4-Ethylenedioxythiophene (manufactured by Bayer AG) wasdissolved in a 75% IPA solution of iron(III) dodecylbenzenesulfonate toan equimolar amount, and this solution was dropped or impregnated on thealuminum foil prepared in Example 1. Subsequently, this aluminum foilwas left standing in an atmosphere at about 40° C. for about 60 minutesto allow the oxidative polymerization to proceed.

[0139] Using a thicknessmeter (manufactured by Peacock Corp.:Digitaldial Gage DG-205 (accuracy of 3 μm)) the thickness was measuredby slowly putting the aluminum foil into the measuring part of thethicknessmeter.

[0140] As a result, the thickness (h₃) at the peripheral part shown inthe schematic view of FIG. 4 was 230 μm, the thickness (h₄) of theoverhanging part at the center part was 240 μm, and the difference(h₃−h₄) in the film thickness was −10 μm.

[0141] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for changing the polymerization method. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 1 and 2. TABLE 1Initial Properties Leakage Number of Capacitance Loss Impedance CurrentDefective Short Circuit μF Factor % m Ω μA Ratio Units Example 1 51.90.603 0.008 0.03 0/30 0 Example 2 50.3 0.635 0.013 0.05 0/30 0 Example 350.9 0.624 0.012 0.06 0/30 0 Example 4 49.8 0.653 0.017 0.07 0/30 0Example 5 49.6 0.678 0.019 0.09 0/30 0 Comparative 40.1 1.658 0.045 0.954/30 4 Example 1

[0142] TABLE 2 Reflow Soldering Test Humidity Resistance Test Number ofNumber of Short Short Defective Circuit Leakage Defective Circuit RatioUnits Current Ratio Units Example 1 0/30 0 0.49 0/30 0 Example 2 0/30 00.54 0/30 0 Example 3 0/30 0 0.59 0/30 0 Example 4 0/30 0 0.57 0/30 0Example 5 0/30 0 0.61 0/30 0 Comparative 3/26 3 2.10 3/23 1 Example 1

[0143] From the results of examples 1 to 5 and comparative example 1, itis found out that the solid electrolytic capacitor having the thicknessof solid electlyte layer on the peripheral part of the porousvalve-acting metal substrate larger than the thickness on the centerpart has excellent stability in various fundamental properties such ascapacitance, dielectric loss (tan δ), leakage current and short circuitdefective ratio and also in the reflow soldering heat resistance and themoisture resistance load characteristics.

EXAMPLE 6

[0144] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the oxidizing agent solution for 3 minutes. Thesecapacitor devices were evaluated on the properties in the same manner asin Example 1. The results obtained are shown in Tables 3 and 4.

EXAMPLE 7

[0145] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the oxidizing solution for 4 minutes in Example 1.These capacitor devices were evaluated on the properties in the samemanner as in Example 1. The results obtained are shown in Tables 3 and4.

EXAMPLE 8

[0146] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the monomer solution for 4 minutes in Example 1. Thesecapacitor devices were evaluated on the properties in the same manner asin Example 1. The results obtained are shown in Tables 3 and 4.

EXAMPLE 9

[0147] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by repeating thedipping step and the polymerization step 20 times in Example 1. Thesecapacitor devices were evaluated on the properties in the same manner asin Example 1. The results obtained are shown in Tables 3 and 4.

EXAMPLE 10

[0148] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by repeating thedipping step and the polymerization step 28 times in Example 1. Thesecapacitor devices were evaluated on the properties in the same manner asin Example 1. The results obtained are shown in Tables 3 and 4.

EXAMPLE 11

[0149] The 3 mm×4 mm portion of an etched aluminum foil on whichdielectric film was formed as in example 1 was dipped in 1.5 mol/L of anaqueous ammonium persulfate solution having suspended therein sodiumanthraquinone-2-sulfonate to a concentration of 0.05 mass %, for 5seconds. Subsequently, this aluminum foil was left standing in apolymerization vessel heated at 60° C. and 3,4-ethylenedioxythiophenegas generated by heating at about 80° C. accompanied with nitrogen gaswas introduced to the polymerizing vessel to allow the vapor phasechemical oxidative polymerization to proceed. Subsequently, this etchedaluminum foil was left standing in an atmosphere at 40° C. for 10minutes to dry. By repeating the dipping step and the polymerizationstep 15 times in total, a solid electrolyte layer of the electricallyconducting polymer was formed on the outer surfaces of the etchedaluminum foil. The finally produced polyer was washed with warm water at70° C. and then dried at 100° C. for 30 minutes to complete theformation of the solid electrolyte layer.

[0150] The thickness of the overhanging part of the solid electrolytelayer (h₁) was measured in the same manner as in Example 1 and found tobe 180 μm, the thickness of the constriction portion at the center part(h₂) was 130 μm, and the difference (h₁−h₂) in the film thickness was 50μm.

[0151] Then 30 units of capacitors were fabricated. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 2

[0152] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by repeating thedipping step and the polymerization step 10 times. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 3

[0153] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the oxidizing solution for 10 minutes. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 4

[0154] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by washing thealuminum foil every each dipping step and every each polymerizationstep. These capacitor devices were evaluated on the properties in thesame manner as in Example 1. The results obtained are shown in Tables 3and 4.

COMPARATIVE EXAMPLE 5

[0155] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by setting thepolymerization temperature at 70° C. These capacitor devices wereevaluated on the properties in the same manner as in Example 1. Theresults obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 6

[0156] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the monomer solution for 10 minutes. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 7

[0157] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the oxidizing agent solution for 7 minutes. Thesecapacitor devices were evaluated on the properties in the same manner asin Example 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 8

[0158] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by dipping thealuminum foil in the monomer solution for 7 minutes. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4.

COMPARATIVE EXAMPLE 9

[0159] 30 Units of capacitors were fabricated in the same manner as inExample 1 except for forming the solid electrolyte by repeating thedipping step and the polymerization step 40 times. These capacitordevices were evaluated on the properties in the same manner as inExample 1. The results obtained are shown in Tables 3 and 4. TABLE 3Initial Properties Number of Loss Short- Capacitance Factor* ImpedanceLeakage Defective Circuited μF % Ω Current μA Ratio Products Example 651.7 0.62 0.009 0.03 0/30 0 Example 7 50.3 0.63 0.013 0.04 0/30 0Example 8 51.6 0.61 0.009 0.03 0/30 0 Example 9 50.2 0.62 0.010 0.030/30 0 Example 10 51.9 0.65 0.008 0.08 0/30 0 Example 11 49.3 0.84 0.0190.31 0/30 0 Comparative 42.1 0.76 0.035 0.80 3/30 3 Example 2Comparative 40.8 0.77 0.027 0.69 4/30 4 Example 3 Comparative 30.7 0.790.030 0.89 3/30 3 Example 4 Comparative 36.9 0.74 0.035 0.95 5/30 5Example 5 Comparative 45.2 0.69 0.024 0.11 2/30 1 Example 6 Comparative49.7 0.72 0.032 0.73 2/30 1 Example 7 Comparative 48.0 0.69 0.022 0.101/30 1 Example 8 Comparative 51.7 0.78 0.010 0.85 4/30 2 Example 9

[0160] TABLE 4 Moisture Resistance Test Reflow Soldering Test Number ofNumber of Leakage Defec- Short- Defective Short-circuited Current tiveCircuited Ratio Products μA Ratio Products Example 6 0/30 0 0.53 0/30 0Example 7 0/30 0 0.60 0/30 0 Example 8 0/30 0 0.46 0/30 0 Example 9 0/300 0.71 0/30 0 Example 10 0/30 0 0.85 0/30 0 Example 11 0/30 0 0.87 0/300 Comparative 1/28 1 2.10 1/27 1 Example 2 Comparative 1/29 0 1.75 2/281 Example 3 Comparative 2/27 1 3.57 4/25 2 Example 4 Comparative 2/28 13.45 3/26 2 Example 5 Comparative 1/28 0 1.12 2/27 1 Example 6Comparative 3/29 1 1.02 3/28 2 Example 7 Comparative 1/28 0 1.52 2/28 1Example 8 Comparative 2/28 1 2.12 3/27 2 Example 9

[0161] As seen from Tables 3 and 4 which show the results of examples 6to 10 and comparative examples 2 to 9, and Tables 1 and 2 which show theresults of foregoing examples 1 to 5 and comparative examples 1, a solidelectrolytic capacitor device exhibiting excellent results in theinitial basic properties such as capacitance, dielectric loss (tan δ),impedance and leakage current and also in the stability to the reflowsoldering heat resistance test and the moisture resistance load test isobtained in the case where the dipping time is less than 5 minutes withvarious monomer solutions used (the dipping time is 5 seconds with3,4-ethylenedioxythiophene in Example 1, with dihydroisothianaphthene inExample 2, with pyrrole in Example 3, with furan in Example 4 or withaniline in Example 5 and the dipping time is 4 minutes with3,4-ethylenedioxythiophene in Example 8), in the case where the dippingtime in the oxidizing agent-containing solution is less than 5 minutes(Examples 6 to 7), and in the case where the alternate dipping in themonomer solution and in the acid solution is repeated from 15 to 30times (Examples 9 to 10). On the other hand, in the case where thedipping time in the monomer solution exceeds 5 minutes (ComparativeExamples 6 and 8), in the case where the dipping time in the oxidizingagent solution exceeds 5 minutes (Comparative Examples 3 and 7), in thecase where the frequency of alternate dipping operations in the monomersolution and in the oxidizing agent solution is small (ComparativeExample 2) or too large (Comparative Example 9), in the case where thewashing is performed after each polymerization by the alternate dipping(Comparative Example 4) and in the case where the polymerizationtemperature exceeds 60° C. (Comparative Example 5), the initial basicproperties or the reflow soldering heat resistance and moistureresistance load characteristics are not stable.

Industrial Applicability

[0162] The solid electrolytic capacitor where solid electrolyte isformed at the cut end part and at the masking part to a larger thicknessthan in other parts is improved in the adhesion of the solid electrolyteformed on the valve acting metal oxide film and highly stabilized invarious basic properties such as capacitance and dielectric loss (tan δ)and also in the reflow soldering heat resistance and moisture resistanceload characteristics.

[0163] These solid electrolytic capacitor can be obtained by formingelectrically conducting polymer on a dielectric film, by specifying timefor dipping of surface of a valve actiong metal porous body with asolution containing a monomer and with a solution containing anoxidizing agent, time for vaporization of the solvent of the solutioncontaining a monomer, and polymerization conditions after dipping withthe solution containing an oxidizing agent. In this process, the metalsurface is alternately dipped with the solution containing a monomer andwith the solution containing an oxidizing agent, each in a predeterminednumber of times and washed at the final stage, whereby an electricallyconducting polymer having a layer structure (lamellar structure orfibril structure) which has excellent properties can be obtained.

What is claimed is:
 1. A solid electrolytic capacitor comprising aporous valve-acting metal substrate having on the surface thereof adielectric film and having provided on the dielectric film a solidelectrolyte of an electrically conducting polymer obtainable byoxidation-polymerizing a monomer of organic polymer using an oxidizingagent, wherein the thickness of the solid electrolyte layer in theperipheral part of the substrate is larger than the thickness of thesolid electrolyte layer in the center part of the substrate.
 2. A solidelectrolytic capacitor comprising a porous valve-acting metal substratehaving on the surface thereof a dielectric film and having provided onthe dielectric film a solid electrolyte of an electrically conductingpolymer obtainable by oxidation-polymerizing a monomer of organicpolymer using an oxidizing agent, said valve-acting metal poroussubstrate being cut into a predetermined shape, wherein the thickness ofthe solid electrolyte layer in the periphery of the cut surface of thesubstrate is larger than the thickness of the solid electrolyte layer inthe center part of the substrate.
 3. A solid electrolytic capacitorcomprising a porous valve-acting metal substrate having on the surfacethereof a dielectric film and having provided on the dielectric film asolid electrolyte of an electrically conducting polymer obtainable byoxidation-polymerizing a monomer of organic polymer using an oxidizingagent, said valve-acting metal porous substrate being cut into apredetermined shape, wherein the thickness of the solid electrolytelayer in the periphery of the cut surface of the substrate and in themasking boundary part is larger than the thickness of the solidelectrolyte layer in the center part of the substrate.
 4. The solidelectrolytic capacitor as claimed in any one of claims 1 to 3, whereinthe partiality in the thickness of said solid electrolyte is created bythe solution chemical oxidative polymerization or vapor phase chemicaloxidative polymerization of a monomer of organic polymer performed onsaid valve-acting metal substrate having thereon a dielectric film. 5.The solid electrolytic capacitor as claimed in claim 4, wherein thepartiality in the thickness of said solid electrolyte is created byrepeating an operation of alternately dipping said valve-acting metalsubstrate having thereon a dielectric film in a solution containing amonomer of organic polymer and in a solution containing an oxidizingagent.
 6. The solid electrolytic capacitor as claimed in any one ofclaims 1 to 5, wherein the porous valve-acting metal substrate has aplate- or foil-like shape.
 7. The solid electrolytic capacitor asclaimed in claim 6, wherein the solid electrolyte is formed such thatthe center part of the porous valve-acting metal substrate has a guitar-or gourd-like cross section in the longitudinal direction and in thetransverse direction.
 8. The solid electrolytic capacitor as claimed inclaim 7, wherein in the cross section of the center part of thesubstrate on which a solid electrolyte layer is formed, the differencebetween the maximum thickness and the minimum thickness is from 0 to 200μm.
 9. The solid electrolytic capacitor as claimed in any one of claims1 to 8, wherein the porous valve-acting metal is a simple metal selectedfrom aluminum, tantalum, niobium and titanium, or an alloy thereof. 10.The solid electrolytic capacitor as claimed in any one of claims 1 to 9,wherein the monomer of organic polymer for forming the electricallyconducting polymer is a compound containing a 5-member heterocyclicring, or a compound having an aniline skeleton.
 11. The solidelectrolytic capacitor as claimed in claim 10, wherein the compoundcontaining a 5-member heterocyclic ring is a compound having a thiopheneskeleton or a polycyclic sulfide skeleton.
 12. The solid electrolyticcapacitor as claimed in claim 11, wherein the monomer compound having athiophene skeleton is 3-ethylthiophene, 3-hexylthiophene,3,4-dimethylthiophene, 3,4-methylenedioxythiophene, and3,4-ethylenedioxythiophene.
 13. A solid electrolytic multilayercapacitor obtainable by stacking a plurality of sheets of the solidelectrolytic capacitor as claimed in any one of claims 1 to
 12. 14. Amethod for producing a solid electrolytic capacitor, comprisingproviding a solid electrolyte of an electrically conducting polymer on adielectric film on the surface of a porous valve-acting metal substrateby the oxidative polymerization of a monomer of organic polymer using anoxidizing agent, wherein the solid electrolyte layer is formed such thatthe thickness in the peripheral part of the substrate is larger than thethickness in the center part of the substrate.
 15. The method forproducing a solid electrolytic capacitor as claimed in claim 14, whereinthe partiality in the thickness of said solid electrolyte is created bythe solution chemical oxidative polymerization or vapor phase chemicaloxidative polymerization of a monomer of organic polymer performed onsaid valve-acting metal substrate having thereon a dielectric film. 16.The method for producing a solid electrolytic capacitor as claimed inclaim 15, wherein the partiality of the thickness of said solidelectrolyte is created by repeating an operation of alternately dippingsaid valve-acting metal substrate having thereon a dielectric film in asolution containing a monomer of organic polymer and in a solutioncontaining an oxidizing agent.
 17. The method for producing a solidelectrolytic capacitor as claimed in claim 16, wherein the substrate isalternately dipped in each solution for less than 5 minutes.
 18. Themethod for producing a solid electrolytic capacitor as claimed in claim16, wherein the alternate dipping is repeated from 15 to 30 times. 19.The method f or producing a solid electrolytic capacitor as claimed inclaim 16, wherein said valve acting metal substrate is left standing inair for from 5 seconds to 15 minutes after the dipping in the solutioncontaining a monomer.
 20. The method for producing a solid electrolyticcapacitor as claimed in claim 16, wherein said valve acting metalsubstrate is left standing in air for from 10 seconds to 15 minutesafter the dipping in said solution containing an oxidizing agent. 21.The method for producing a solid electrolytic capacitor as claimed inclaim 19 or 20, wherein said valve acting metal substrate is leftstanding in air at a temperature of 0 to 60° C.
 22. The method forproducing a solid electrolytic capacitor as claimed in claim 16, whereinafter the step of dipping said valve acting metal substrate alternatelyin the solution containing a monomer and in the solution containing anoxidizing agent to perform the polymerization, said valve acting metalsubstrate is washed.
 23. A method for producing a solid electrolyticcapacitor, comprising forming a solid electrolyte layer of anelectrically conducting polymer on the surface of a valve acting metalsubstrate having thereon a dielectric film porous body, using a solutioncontaining a monomer capable of forming an electrically conductingpolymer under the action of an oxidizing agent, and a solutioncontaining an oxidizing agent, which has a step of dipping said valveacting metal substrate alternately in the solution containing a monomerand in the solution containing an oxidizing agent each for less than 5minutes to perform the polymerization.
 24. A method for producing asolid electrolytic capacitor, comprising forming a solid electrolytelayer of an electrically conducting polymer on the surface of a valveacting metal substrate having thereon a dielectric film porous body,using a solution containing a monomer capable of forming an electricallyconducting polymer under the action of an oxidizing agent, and asolution containing an oxidizing agent, wherein a step of dipping saidvalve acting metal substrate alternately in the solution containing amonomer and in the solution containing an oxidizing agent each for lessthan 5 minutes is repeated from 15 to 30 times to perform thepolymerization.
 25. The method for producing a solid electrolyticcapacitor as claimed in claim 23 or 24, wherein said valve acting metalsubstrate is left standing in air for from 5 seconds to 15 minutes afterthe dipping in the solution containing a monomer.
 26. The method forproducing a solid electrolytic capacitor as claimed in any one of claims23 to 25, wherein said valve acting metal substrate is left standing inair for from 10 seconds to 15 minutes after the dipping in said solutioncontaining an oxidizing agent.
 27. The method for producing a solidelectrolytic capacitor as claimed in claim 25 or 26, wherein said valveacting metal substrate is left standing in air at a temperature of 0 to60° C.
 28. The method for producing a solid electrolytic capacitor asclaimed in any one of the claims 23 to 27, wherein after repeating thestep of performing polymerization, said valve acting metal substrate iswashed.
 29. The method for producing a solid electrolytic capacitor asclaimed in any one of claims 23 to 28, wherein a part of the solidelectrolyte layer formed of an electrically conducting polymer has alamellar structure or a fibril structure.
 30. The method for producing asolid electrolytic capacitor as claimed in any one of claims 23 to 29,wherein the monomer for forming an electrically conducting polymer is acompound containing a heterocyclic 5-membered ring.
 31. The method forproducing a solid electrolytic capacitor as claimed in any one of claims23 to 30, wherein the monomer for forming an electrically conductingpolymer is a compound having an aniline skeleton.
 32. The method forproducing a solid electrolytic capacitor as claimed in claim 30, whereinthe compound containing a heterocyclic 5-membered ring has a thiopheneskeleton.
 33. The method for producing a solid electrolytic capacitor asclaimed in claim 30, wherein the monomer for forming an electricallyconducting polymer is selected from the group consisting of3-ethylthiophene, 3-hexylthiophene, 3,4-dimethylthiophene,3,4-methylenedioxythiophene, 3,4-ethylenedioxythiophene,1,3-dihydroisothianaphthene, and 3,4-ethylenedioxyfurane.
 34. The methodfor producing a solid electrolytic capacitor as claimed in any one ofclaims 23 to 33, wherein said valve acting metal is a single metalselected from aluminum, tantalum, niobium and titanium, or an alloythereof.
 35. A solid electrolytic capacitor obtained by the productionmethod as claimed in claim
 23. 36. A solid electrolytic capacitorobtained by the production method as claimed in claim
 24. 37. A solidelectrolytic capacitor obtained by the production method as claimed inany one of claims 25 to 34.